1. Field of the Invention
The present invention relates to a drive apparatus configured to constant-current-control a winding current of a stepping motor.
2. Description of the Related Art
The use of a stepping motor provides an advantage of enabling an open-loop control including a position control based on a pulse count of command pulses (a drive control pulse signal) and a speed control based on an inter-pulse period of the command pulses. On the other hand, however, there is a disadvantage that if a load torque exceeds an allowable output torque range of the stepping motor, the stepping motor may fall into a state called step-out, in which the stepping motor cannot be synchronized with the command pulses and rotate normally. In order to prevent the step-out, a load torque necessary for a real machine is usually set within an output torque range in which the stepping motor does not lose steps. Then, in a constant-current-controlling circuit for driving the stepping motor, it is a general manner to set a current value for generating an output torque within the output torque range free from the step-out. In a case where the load torque changes depending on various conditions, a certain margin (called torque margin) is set so as to generate an output torque prepared for the change in load torque. However, providing a sufficient torque margin may produce a too-large output torque with respect to the load torque so that the rotor undergoes a repeated phenomenon of rotating too much and turning back in response to a single command pulse, resulting in vibration. Aimed at solving this problem, there have been proposed a method involving switching a current setting value according to a drive situation (an expected load state) (Japanese Patent Application Laid-Open No. H04-265698) and a method involving switching a phase exciting operation according to the drive situation (Japanese Patent Application Laid-Open No. 2006-136149).
Meanwhile, in order to solve the disadvantage inherent in the open-loop control, there has been proposed a stepping motor drive apparatus employing a closed-loop control using a position/speed detecting unit (encoder, resolver, etc.) (Japanese Patent Application Laid-Open No. H06-225595 and Japanese Patent Application Laid-Open No. H08-182392). Further, aimed at solving the problems of control delay and poor stability, which are the disadvantages of the closed-loop control, it has been also proposed to switch the motor control from the open-loop control for normal use to the closed-loop control only when the motor is liable to lose steps (Japanese Patent Application Laid-Open No. H11-113289).
The technologies described in Japanese Patent Application Laid-Open No. H04-265698 and Japanese Patent Application Laid-Open No. 2006-136149 set a hard-coded value for the motor-applied load torque with respect to a maximum torque expected in an operation sequence including an accelerating period, a constant-speed period, and a decelerating period. Those technologies set the hard-coded value so as to reduce the loss, vibration, and heat generation, merely aiming at the improvement within an expected range including a certain margin. Therefore, even switching the current setting value or the phase exciting operation depending on the drive situation cannot avoid a case difficult to respond to unexpected load fluctuations.
To attain a margin-less drive method unlike using the hard-coded value, Japanese Patent Application Laid-Open No. H06-225595, Japanese Patent Application Laid-Open No. H08-182392, and Japanese Patent Application Laid-Open No. H11-113289 have proposed a drive method incorporating the closed-loop control, which employs a synchronous motor drive method using a sensor for detecting a position/speed. However, the additional position/speed detecting unit for the closed-loop control is responsible for an increase in apparatus size and cost.